For a number of years, step drill bits have been commonly used, for example in the electrical industry, for cutting holes in panels made of sheet metal. One of the advantages that these step bits provide is that they can be used to cut a variety of holes of different diameters without requiring different drill bits to be mounted to and dismounted from a chuck of a powered tool. This, of course, saves time and money in the field.
A conventional step drill bit 10 is shown in FIGS. 1-4. For the purposes of reference and ease of discussion, the end 12 of the drill bit 10 which first contacts the workpiece will be referred to as the front, tip or forward end of the drill bit 10 and the end 14 of the drill bit 10 which is received by the chuck of the tool will be referred to as the back or rear, or rearward end of the drill bit 10. A central axis 16, about which the step drill bit 10 rotates, extends from the front end 12 to the rear end 14 of the drill bit 10.
A main body 18 of the step drill bit 10 is generally conically-shaped and extends from the tip 12 to a rear wall 20. A series of alternating cutting surfaces 22 and transition surfaces 24 are provided along the length of the main body 18. Each of the cutting surfaces 22 is generally cylindrically-shaped and has a lip or cutting edge 31 which is capable of cutting a hole of a predetermined diameter and a forward edge 33 along the circumference of the cutting surface 22. In the rearward direction, the diameter of each cutting surface 22 at the forward edge 33 increases relative to the diameter of the previous cutting surface 22 at the forward edge 33.
As best shown in FIG. 2, two diametrically-opposed flutes 26 are provided in the main body 18 of the step drill bit 10. Each flute 26 extends from the tip 12 to the rear wall 20 of the main body 18. The flutes 26 define first and second sections 28, 30 of the main body 18 extending from the tip 12 to the rear wall 20 of the main body 18. Each flute 26 intersects each cutting surface 22 so as to form two cutting segments 22a, 22b from each cutting surface 22.
In order to properly cut a hole, reliefs are required between the drill bit 10 and the workpiece to be cut. More specifically, reliefs are required between the cutting surfaces 22 of the drill bit 18 and the workpiece. These reliefs include axial relief, diametral relief and radial relief. FIGS. 1 and 2 demonstrate each of the reliefs incorporated into the step drill bit 10. As shown in FIG. 1, an axial relief A provides clearance along the axis 16 for the cutting surface 22 of the step drill bit 10 as the drill bit 10 is rotated. The axial relief A is defined by the angle between a plane perpendicular to the axis 16 and the forward edge 33 of the cutting surface 22. As shown, a minimum axial relief of 1° is provided. A diametral relief B is shown in FIG. 1 which provides clearance behind forward edges 33 of the cutting surfaces 22 of the step drill bit 10. The diametral relief B is defined by an angle between a line parallel to the axis 16 and a line tangent to the cutting surfaces 22. Thus, as the step drill bit 10 moves forward into the workpiece, the forward edge 33 of the cutting surface 22 contacts the workpiece. Immediately behind the forward edge 33, the diameter of the step drill bit 10 is reduced to ease the forward movement of the step drill bit 10 further into the workpiece. As shown, a diametral relief B of 0.5° is provided. FIG. 2 demonstrates the radial relief C provided by the step drill bit 10. As noted, each cutting surface 22 is divided in two segments 22a, 22b by the flutes 26. Each segment includes a leading edge 31 and a trailing edge 32. Along each segment 22a, 22b, the radius of step drill bit 10 decreases such that the radius of the drill bit 10 at the cutting edge 31 is larger than the radius of the drill bit at the trailing edge 32. Thus, the radial relief C is provided by the difference in the radius of the cutting surface proximate the cutting edge 31 and the radius of the cutting surface proximate the trailing edge 32. This radial relief C eases the rotation of the step drill bit 10 within the workpiece. As shown, the radial clearance C is 0.009 in/in.
Commonly, step drill bits, such as the one shown in FIGS. 1-3, are formed by first turning the general profile of the step bit and then grinding the necessary axial, diametral, and radial clearances A, B, C. A disadvantage of this technique is that a considerable amount of force is necessary to grind all three types of clearances onto the step drill bit. As a result, the grinding time necessary to make the step drill bit is relatively long and is therefore costly.
Another disadvantage is that it is difficult to grind multiple cutting surfaces 22 on the first section 28 of the main body 18 without damaging cutting surfaces 22 on the second section 30 of the main body 18. Grinding of each section 28, 30 is preformed by rotating a grinding wheel proximate the perimeter of the step drill bit 10 to shape the step drill bit 10, along with the desired clearances. In order to grind the perimeter of the step drill bit 10, the step drill bit 10 is rotated relative to the grinding wheel. Because the diameters of the cutting surfaces 22 proximate the tip 12 are smaller than the diameter of the cutting surfaces 22 proximate the rear wall 20, grinding of the cutting segments 22a of the first section 28 proximate the tip 12 will be completed prior to grinding of the cutting segments 22a of the first section 28 proximate the rear wall 20. In order to complete grinding of the segments 22a proximate the rear wall 20, rotation of the step drill bit 10 relative to the grinding wheel must continue. As a result, cutting segments 22b of the second section 30 come close to the grinding wheel and in some instances the grinding wheel inadvertently “nicks” cutting segments 22b on the second section 30 of the step drill bit 10. FIG. 3 illustrates “nicks” 36 on the cutting surfaces 22 of the second segments 22b of the step drill bit 10.
Accordingly, there exists a need to improve the method of manufacturing drill bits so that they can be made more time and cost efficiently, while still providing the necessary clearances on the drill bit so that it can cut effectively. The present invention provides a spiral drill bit which overcomes the problems presented in the prior art and which provides additional advantages over the prior art, such advantages will become clear upon a reading of the attached specification in combination with a study of the drawings.